Andrew Harnik, Associated Press
President Barack Obama arrives to chair the Leaders’ Summit on Countering ISIL and Countering Violent Extremism, Tuesday, Sept. 29, 2015, at the United Nations headquarters.

UNITED NATIONS — President Barack Obama on Tuesday pledged all possible tools — military, intelligence and economic — to defeat the Islamic State group, but acknowledged the extremist group has taken root in Syria and Iraq, is resilient and continues to expand.

Obama spoke as chairman of a U.N. gathering of world leaders working to expand the battle against terrorism, a day after he and the leaders of Russia, China and Iran addressed the General Assembly on its 70th anniversary. The fight against terrorism, particularly in Syria, has seized the attention of top officials, but there has been no overall agreement on how to end the conflict there.

"I have repeatedly said that our approach will take time. This is not an easy task," he cautioned, while adding that he was "ultimately optimistic" the brutal organization would be defeated.

"This is not a conventional battle, this is a long-term campaign not only against this particular network but against their ideology," he said.

The Islamic State is attracting fighters from around the world, prompting fears they will return to their home countries to launch attacks. And the fight has been complicated by a Russian military buildup in Syria in support of President Bashar Assad. Russian President Vladimir Putin suggested Monday that Russia could launch airstrikes against the militants in Syria, if sanctioned by the United Nations or requested by Damascus.

Obama and Putin are at odds over Russian involvement because Washington has said Assad must be removed from power.

Reflecting the divide, an official with the Russian delegation said Moscow would take part in the Obama-led event only by a lower-level official, U.N. deputy ambassador Evgeny Zagaynov. Russia on Wednesday will chair its own meeting on countering extremism as this month's U.N. Security Council president.

Obama and Putin laid out competing visions for Syria during their speeches to the opening meeting of the annual U.N. General Assembly on Monday.

Also set for Tuesday are high-level meetings on the humanitarian consequences of the war in Syria and the wider refugee and migrant crisis that is the largest since the upheaval of World War II. Refugees have swept into Europe in waves and already are jammed into camps in Jordan, Turkey and Lebanon.